1. Field of the Invention
The present invention relates to a scanning optical apparatus and an image forming apparatus using the same. For example, the present invention is particularly suitably usable in an image forming apparatus such as a laser beam printer (LBP), a digital copying machine or a multifunction printer, having an electrophotographic process.
2. Description of the Related Art
Various types of scanning optical apparatuses for a laser beam printer (LBP) having an electrophotographic process have conventionally been proposed (see Japanese Patent No. 2003-241126).
FIG. 8 is a sectional view (main scanning sectional view) in the main scanning direction of a main portion of a conventional scanning optical apparatus, and FIG. 9 is a sectional view (sub scanning sectional view) of the main portion of FIG. 8 in the sub scanning direction.
FIGS. 8 and 9 illustrate a light source unit 1 which includes a semiconductor laser having a single light emitting portion (light emission point).
A light flux emerged from the light source unit 1 is shaped by an aperture stop 3 and is transformed into parallel light beams by means of a collimator lens 2. Then, the parallel light beams are converged by a cylindrical lens 4 only in the sub scanning direction.
The parallel light beams converged by the cylindrical lens 4 only in the sub scanning direction are then imaged into a focal line shape (line shape) extending in the main scanning direction, adjacent to a deflection surface 5a of an optical deflector (rotationed polygon mirror) 5 which is a deflection unit.
The elements of collimator lens 2 and cylindrical lens 4 described above are components of an incident optical system LA.
The parallel light beams deflectively scanned by the rotational polygon mirror 5 which is rotating at a constant angular speed in the direction of an arrow 5b in the drawing, are collected into a spot shape on a surface to be scanned 7 including a photosensitive drum, by means of an imaging lens 6a constituting an imaging optical system 6. The spot-shaped light flux scans the surface to be scanned 7 at a constant speed in the direction of an arrow 7b in the drawing.
A plurality of deflection surfaces 5a constituting the rotational polygon mirror 5 are formed at the right angle to the main scanning direction. However, due to a processing error or the like, these surfaces may have a tilt in the sub scanning direction, which causes displacement of the spot imaged on the surface to be scanned 7 in the sub scanning direction.
In order to correct such a displacement, as the imaging optical system 6, an optical face tangle error compensation system based on an anamorphic system is used, in which the vicinity of the deflection surface 5a and the surface to be scanned 7 are brought into a conjugate relationship with each other within a sub scanning section. There is known a scanning optical apparatus using such an optical face tangle error compensation system (see Japanese Patent No. 2003-21126).
In the optical face tangle error compensation system, the surface to be scanned and a plane in the vicinity of the deflection surface parallel to the surface to be scanned are brought into a conjugate relationship with each other within the sub scanning section.
However, a rotational polygon mirror has its deflection surface not placed on a rotational axis, and hence a position of the deflection surface is displaced in an optical axis direction integrally with rotational scanning.
In other words, it is only at a predetermined rotation angle of the rotational polygon mirror that the deflection surface of the rotational polygon mirror is made conjugate (coincide) with a conjugate point of the surface to be scanned. An intersection point between an incident light flux and the deflection surface is displaced in a direction away from the surface to be scanned accompanying the rotation of the rotational polygon mirror. The intersection point is most distant from the surface to be scanned at a scan center, and displaced in a direction approaching the surface to be scanned when the rotational polygon mirror further rotates.
The position of the deflection surface exhibits a quadratic function change with respect to a main scanning image height. Normally, therefore, the apparatus is designed such that a conjugate relationship is established at two points of a scanning region, i.e., at both ends. As a result, an optical face tangle error cannot be suppressed in principle in an area of the scanning region outside the area where the conjugate relationship is established.
On the other hand, attaching importance to optical face tangle error compensation performance, a conjugate relationship can be established throughout the scanning region. However, such designing causes great curvature of an imaging position of a spot of the light flux in the sub scanning direction of the surface to be scanned 7 due to an image height, which deteriorates basic performance.
Meanwhile, a conjugate relationship is uniquely determined by paraxial performance (object point position or lens focal length) of the imaging optical system. On the other hand, a best imaging position (focusing position) of a spot can be controlled to a certain extent based on a spherical aberration amount of the imaging optical system.
Paraxial performance may be determined so that the deflection surface can be made to have a conjugate relationship with portions on the surface to be scanned at all image heights, and controlling of a focusing position may be performed by using an aspherical coefficient of fourth or higher order in a surface shape of the sub scanning section of the imaging optical system.
In this manner, amounts of generated spherical aberration can be made different between the main scanning center and the end in the main scanning direction.
This way, positions of the deflection surface at all the image heights can be made coincide with each other on the surface to be scanned 7. In other words, in the position of the main scanning center, an aspherical shape is formed so that a curvature radius of a peripheral portion in the sub scanning direction of the imaging optical system can be larger than that of a central portion in the sub scanning direction.
In the end position of the main scanning direction, a curvature radius of a lens surface of the peripheral portion in the sub scanning direction of the imaging optical system is set smaller than that of the central portion in the sub scanning direction, and the lens surface is formed into an aspherical shape.
A relationship for changing the curvature radius in the sub scanning direction by the aspherical effect is as described above in the case of a scanning optical system of an under-filled type. In the case of a scanning optical system of an over-filled type, however, this relationship between the main scanning center and the main scanning end is reversed.
That is, in the position of the main scanning center, a curvature radius of a surface of the sub scanning direction peripheral portion of the imaging optical system is set smaller than a curvature radius of a surface of the sub scanning center, and the lens surface is formed into an aspherical shape. In the position of the main scanning end, a curvature radius of a surface of the sub scanning direction peripheral portion of the imaging optical system is set larger than a curvature radius of a surface of the sub scanning center, and the lens surface is formed into an aspherical shape.
Determining the aspherical shape of the surface of the imaging optical system as described above enables focusing while maintaining a conjugate relationship at all the image heights.
The following problems occur when the aforementioned designing is performed by forming the lens surface into an aspherical shape on the sub scanning section.
There is assumed a case where a manufacturing error or an assembling error of optical components constituting the imaging optical system causes a light flux made incident on the imaging optical system to be shifted in height in a vertical direction. The incident light flux passes through a position distant from an optical axis in the sub scanning section of the imaging optical system, and hence upper and lower ends of the light flux pass through positions asymmetrical from the optical axis.
That is, the upper and lower ends of the light flux are set at incident angles asymmetrical to a surface normal of a sectional shape, causing many coma aberrations. The aspherical sectional shape causes a change in curvature radius as a distance from the optical axis increases in the sub scanning direction, resulting in greater coma aberrations as compared with a case where a conventional spherical lens is used. As a consequence, a side lobe of the spot becomes larger, disabling good image formation.